The NIR region of the electromagnetic spectrum lies between 700 and 2500 nm. Materials having high reflectance and reduced absorption in this range may be advantageous in many applications. For instance, products made from such materials tend to remain cooler under solar illumination and lower temperatures can result in lower thermal degradation, improved durability, greater comfort, lower air conditioning costs, and reduced environmental impact.
A current environmental focus (and cost factor) is to reduce the amount of air conditioning needed to cool buildings. One way to reduce air conditioning costs is to use roofing products that reflect solar energy. The US Environmental Protection Agency (EPA) Energy Star Initiative requires steep-slope (pitched) residential roofing to have a minimum Total Solar Reflectance (TSR) of 25%. Lighter coloured products may be able to meet this minimum, but by their nature, dark or intensely coloured products may not be able to so and tend to have a TSR well below 25% such as 10% or less. This can create a problem for those who find dark or intense colours aesthetically pleasing, but want the advantages of a higher TSR.
High solar reflectance may be achieved in different ways. For instance, items with white outer surfaces may have high solar reflectance, but if a colour is desired this approach is unsatisfactory. Alternatively, high solar reflectance may be achieved by combining conventional TiO2 pigments with non-NIR absorbing coloured pigments and dyes. This approach is also limited because the levels of conventional TiO2 pigment required to give the desired levels of solar reflection will necessarily result in relatively pale colours. Therefore, darker or more intense colours are not possible in such a reflective formulation. In yet another alternative, a white layer having a high solar reflectance may be applied to an item, which is followed by a layer containing NIR-transparent coloured pigments. The pigmented overcoat does not reflect or absorb NIR radiation. This system too is not ideal because it takes time to apply the two different coats, they can, if not properly applied, result a “patchy” appearance with the white undercoat showing through portions of the coloured overcoat, and the colour may lighten over time as the overcoat weathers away exposing more undercoat.
Thus, there is a need for a high total solar reflective material that is available in a wide range of darker or more intense colours than would be otherwise achievable for a given solar reflectance. Such colours include mid-tones and even darker/more intense pastels. Furthermore, there is a need for a one-coat system to apply such solar-reflective coloured materials, which can be used in a range of applications including roofing surfaces, plastic items, road surfaces and paints. In this way, consumers could then have items they want with both the desired coloured appearance and good total solar reflection. Those items could then contribute to a cooler living environment and/or reduced air conditioning energy usage, thermal degradation, environmental footprint, and/or contribution to global warming.
Additionally, items exposed to the sun may not be photostable and can prematurely deteriorate. Such items including paints, plastics products, roofing products, and ground covering products, may contain titanium dioxide. Although titanium dioxide itself does not degrade, the extent to which an item containing titanium dioxide degrades may depend upon the photocatalytic activity of the titanium dioxide pigment used in the item.
For example, and without being bound by theory, if a titanium dioxide crystal absorbs UV light, it is thought that an electron is promoted to a higher energy level (the conductance band) and moves through the lattice. The resulting vacancy or “hole” in the valence band also effectively ‘moves’. If these mobile charges reach the crystal surface, they can be transferred to the medium of the titanium dioxide containing article (e.g. the resinous medium of paint), and produce free radicals which degrade the medium.
Thus, there is a need for titanium dioxide particles having ultra-low photocatalytic activity. Such titanium dioxide particles may then be used to improve the lifetime of items exposed to the sun. For example, such titanium dioxide particles may be used in combination with highly photostable resins, paint binders, and the like, to lengthen the overall lifetime of a sun-exposed item.